The present invention relates to a joined scintillator block body for radiation detector used in radiologic diagnostic apparatus such as a positron emission tomography apparatus (e. g., positron computer tomography; positron CT).
The positron CT is an imaging apparatus for forming tomographic images of a subject to be examined by administering a radiation source capable of emitting positrons to the subject, detecting .gamma.-rays emitted upon the extinction of the positrons with radiation detectors arranged around the subject in the form of a ring while rotating and scanning the detectors to obtain information from multiple directions and processing the information to give a tomographic image of the subject.
There has been used a detector which comprises a photomultiplier tube and a scintillator divided into a plurality of divisions, i. e., scintillator chips and connected to an entrance window of the photomultiplier tube, as such a radiation detector used in the positron CT. The .gamma.-rays emitted from the radiation source in the subject to be examined is in order made incident upon the scintillator chips of the radiation detector, to thus cause emission of scintillation light rays. The scintillation light rays are photoelectrically converted into electric signals by the photomultiplier tube and the latter outputs the corresponding successive electrical signals. Thus, the finer each divided scintillation chip, the higher the space resolution of the resulting tomographic image.
An example of such scintillator is shown in FIG. 3 wherein a scintillator block 2 is provided with a plurality of deep grooves 5 arranged in constant intervals and the grooves are filled with a reflecting material 6. Thus the scintillator block 2 is divided into a plurality of scintillator chips 2.sub.1, 2.sub.2, .... In order to reduce the loss of radiation signals while making the most use of the photomultiplier tube 1, it is preferred to use the entire plane 1a of incidence of the photomultiplier tube 1. To this end, the entire plane 1a of incidence of the surface of the tube must be covered with a large scintillator block on the order of not less than 50 mm. However, it is impossible to obtain a single crystal for scintillator having such a large size from the technical standpoint. For this reason, there has been used a joined scintillator block body 25 which comprises a plurality of scintillator blocks joined to one another through layers of a reflecting material, for instance, a scintillator block 2 divided into a plurality of scintillator chips 2.sub.1. 2.sub.2 .... and a scintillator block 3 likewise divided into a plurality of small scintillator chips 3.sub.1, 3.sub.2, ... are bonded together through layers of a reflecting material 6.
If such a joined scintillator block body 25 is employed, the .gamma.-rays made incident upon the scintillator chip 2.sub.3 located in the middle of the scintillator block 2 cause the emission of scintillation light rays, the maximum quantity of light is made incident upon the portion on the photomultiplier tube 1 just under the scintillator chip 2.sub.3. while attenuated light rays are made incident upon other portions on the photomultiplier tube 1 below the scintillation chips 2.sub.2 and 2.sub.4 approximately symmetrically with respect to the portion just behind the chip 2.sub.3 and the tube 1 outputs detected signals in proportion to the dose of the .gamma.-rays. On the other hand, .gamma.-rays made incident upon the scintillator chip 2.sub.1 adjacent to the scintillator block 3 cause the emission of scintillation light rays which are made incident upon the portions on the photomultiplier tube 1 just below the scintillator chip 2.sub.1 and below the chip 2.sub.2, but are not made incident upon the portions on the tube just below the chip 3.sub.1 because the reflecting material 6 extends over entire joined interface between the scintillator blocks 2 and 3. For this reason, the quantity of positron-detection signal (photomultiplier counts) detected by the photomultiplier tube 1 is reduced at such joined portion. For instance, there is observed a portion at which the photomultiplier counts are low in the middle of the plane 1a of light incidence as shown in FIG. 4. This requires the incorporation of a corrective circuit into the detection circuit.